Hydrogen used for fuel cells and the like was conventionally generated by mixing steam with a hydrocarbon type fuel such as methane, propane, gasoline or kerosine, an alcohol type fuel such as methanol, or an ether type fuel such as dimethylether and bringing the mixed gas into contact with a heated reforming catalyst.
In general, the hydrocarbon type fuel is reformed at temperatures of 500 to 800° C. while the alcohol type fuel and ether type fuel are reformed at temperatures of 200 to 400° C. CO generates in the reforming; as the reforming temperature is higher, the concentration of CO generating rises. Particularly, when the hydrocarbon type fuel is used, the CO concentration of the reformed gas grows up to about 10% by volume. For this reason, CO and hydrogen are reacted to each other by using a CO shifting catalyst to lower the CO concentration to thousands of ppm to several % by volume.
Moreover, in the case of fuel cells which operate at low temperatures of 100° C. or lower such as solid polymer type fuel cells in vehicle-mounted or domestic uses, there is a possibility that a Pt catalyst used in the electrodes is poisoned with CO contained in the reformed gas. Therefore, the CO concentration needs to be removed to 100 ppm or less, preferably 10 ppm or less, before the reformed gas is supplied to the fuel cell. For this reason, a CO purifying unit in which a catalyst is filled is arranged in a hydrogen purifier and then CO is methanated or selectively oxidized after addition of trace amounts of air, thereby removing CO.
When CO is selectively oxidized for removal with a CO purifying catalyst, a noble metal catalyst such as Pt, Ru, Rh or Pd is mainly used. Oxygen in an amount one to three times larger than CO is required to sufficiently remove CO.
Herein, when the amount of hydrogen to be supplied for changing the amount of power generation of the fuel cell system or when a catalytic activity decreases to a certain degree after the long-duration operation of the apparatus, the CO concentration in the reformed gas changes. It is therefore necessary to detect the CO concentration in order to control the amount of oxygen to be an optimum value.
However, application of a generally-implemented technique for detecting the CO concentration from absorption of ray of light of infrared wavelength due to CO, or technique for detecting the CO concentration from a change in resistance due to absorption of CO, is difficult at present because it does not function stably in the reformed gas or the cost thereof is high.
It has thus been difficult to always keep the oxygen to be supplied to the CO purifying catalyst in an optimum volume. It has also been difficult at the time of start-up of the fuel cell system to determine whether the reformed gas can be supplied to the fuel cell or not even after sufficient removal of CO in the hydrogen purifier.
As thus been described, in the conventional technique, there is no step for detecting the CO concentration which is effective in the reformed gas, low-cost and reliable, making the CO purifying catalyst insufficiently exert the function thereof or necessitating a long-time standby operation of the fuel cell system before the start-up thereof at which the supply of the reformed gas to the fuel cell begins.
Accordingly, it is an object of the present invention to provide a step capable of reliably detecting the CO concentration in the reformed gas at low cost, and to provide a hydrogen purifier with the function of the CO purifying catalyst in full play.